Formation of K-feldspar megacrysts in granodioritic plutons by thermal cycling and late-stage textural coarsening

摘要

K-feldspar megacrysts in granite and granodiorite plutons are generally inferred to be early crystallizing phases (grown to large sizes when the magma was mostly liquid) owing to their large size, euhedral form, and features that suggest deposition by magmatic sedimentation. However, phase equilibrium experiments and natural examples of crystallization and partial melting demonstrate that K-feldspar is one of the last phases to nucleate and that most crystal growth must occur after the magma has exceeded 50% crystallization and is thus largely incapable of flow and sedimentation. Megacryst size distributions, compositions, and textural relationships from the Cretaceous Tuolumne Intrusive Suite, California, reveal that the gradational transition from equigranular to megacrystic granodiorite likely occurred via textural coarsening caused by thermal cycling. Experimental and theoretical studies demonstrate that rising temperature induces relatively more melting in small crystals than in large ones, whereas linear growth rates during cooling are similar. Thus, during thermal cycling material is transferred from small crystals to larger ones. Megacryst growth via thermal cycling during incremental emplacement is consistent with the required late growth of K-feldspar, explains the presence of megacrysts in the inner parts of theTuolumne Intrusive Suite and elsewhere, and may be a common process in formation of megacrystic granitic rocks.